linux-kernel - [PATCH] sched, s390: Fix the fallout of increasing the offset of 'thread_struct' within 'task

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Message-ID: <20150720080032.GA12468@gmail.com>
Date:	Mon, 20 Jul 2015 10:00:32 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Heiko Carstens <heiko.carstens@...ibm.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...nel.org>, Dave Hansen <dave@...1.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: [PATCH] sched, s390: Fix the fallout of increasing the offset of
 'thread_struct' within 'task_struct'


* Heiko Carstens <heiko.carstens@...ibm.com> wrote:

> >     I've tested it on a number of x86 platforms, and I cross-built it to a handful 
> >     of architectures:
> > 
> > 	                                 (warns)               (warns)
> > 	testing     x86-64:  -git:  pass (    0),  -tip:  pass (    0)
> > 	testing     x86-32:  -git:  pass (    0),  -tip:  pass (    0)
> > 	testing        arm:  -git:  pass ( 1359),  -tip:  pass ( 1359)
> > 	testing       cris:  -git:  pass ( 1031),  -tip:  pass ( 1031)
> > 	testing       m32r:  -git:  pass ( 1135),  -tip:  pass ( 1135)
> > 	testing       m68k:  -git:  pass ( 1471),  -tip:  pass ( 1471)
> > 	testing       mips:  -git:  pass ( 1162),  -tip:  pass ( 1162)
> > 	testing    mn10300:  -git:  pass ( 1058),  -tip:  pass ( 1058)
> > 	testing     parisc:  -git:  pass ( 1846),  -tip:  pass ( 1846)
> > 	testing      sparc:  -git:  pass ( 1185),  -tip:  pass ( 1185)
> > 
> >      ... so I hope the cross-arch impact 'none', as intended.
> > 
> >     (by Dave Hansen)
> 
> Unfortunately not true. It breaks the build on s390 since a couple of
> displacements used in asm code now get too large:
> 
> arch/s390/kernel/entry.S:181: Error: operand out of range (0x00000000000018a8 is not between 0x0000000000000000 and 0x0000000000000fff)
> arch/s390/kernel/entry.S:191: Error: operand out of range (0x00000000000018a8 is not between 0x0000000000000000 and 0x0000000000000fff)
> arch/s390/kernel/entry.S:423: Error: operand out of range (0x0000000000001924 is not between 0x0000000000000000 and 0x0000000000000fff)
> arch/s390/kernel/entry.S:437: Error: operand out of range (0x00000000000018e8 is not between 0x0000000000000000 and 0x0000000000000fff)
> arch/s390/kernel/entry.S:438: Error: operand out of range (0x00000000000018e0 is not between 0x0000000000000000 and 0x0000000000000fff)
> arch/s390/kernel/entry.S:439: Error: operand out of range (0x00000000000018f0 is not between 0x0000000000000000 and 0x0000000000000fff)
> make[1]: *** [arch/s390/kernel/entry.o] Error 1
> 
> Let's see how we can fix this.

There's also a traps.c build breakage reported below - and an RFC fix for it.

Thanks,

	Ingo

=========================>

* Guenter <linux@...ck-us.net> wrote:

> Hi,
> 
> Commit 0c8c0f03e3a2 ("x86/fpu, sched: Dynamically allocate 'struct fpu'")
> causes s390 builds in mainline to fail as follows.
> 
> arch/s390/kernel/traps.c: Assembler messages:
> arch/s390/kernel/traps.c:262: Error: operand out of range
> 	(0x00000000000023e8 is not between 0x0000000000000000 and 0x0000000000000fff)
> arch/s390/kernel/traps.c:300: Error: operand out of range
> 	(0x00000000000023e8 is not between 0x0000000000000000 and 0x0000000000000fff)


Yeah, so I'm really out on a limb here as I know next to nothing about s390 
assembly, but the build failure appears to be analogous to the arm64 one: the 
offset of thread_struct fields within task_struct increased due to commit 
0c8c0f03e3a2 ("x86/fpu, sched: Dynamically allocate 'struct fpu'"), which 
increased assembly offsets beyond the limit this instruction can apparently 
encode.

Does the (untested!) patch below help?

It's an equivalent transformation on the C side, but it might cause GCC to 
generate different assembly code, because we now have a temporary variable with 
much smaller offsets.

The code is also a tiny bit cleaner this way, as the 'current->thread.fp_regs' 
pattern isn't repeated twice.

In case this works:

   Signed-off-by: Ingo Molnar <mingo@...nel.org>

Thanks,

	Ingo

================>

 arch/s390/kernel/traps.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/s390/kernel/traps.c b/arch/s390/kernel/traps.c
index 4d96c9f53455..db6f0eec55b5 100644
--- a/arch/s390/kernel/traps.c
+++ b/arch/s390/kernel/traps.c
@@ -251,6 +251,7 @@ int alloc_vector_registers(struct task_struct *tsk)
 
 void vector_exception(struct pt_regs *regs)
 {
+	s390_fp_regs *fp_regs = &current->thread.fp_regs;
 	int si_code, vic;
 
 	if (!MACHINE_HAS_VX) {
@@ -259,8 +260,9 @@ void vector_exception(struct pt_regs *regs)
 	}
 
 	/* get vector interrupt code from fpc */
-	asm volatile("stfpc %0" : "=m" (current->thread.fp_regs.fpc));
-	vic = (current->thread.fp_regs.fpc & 0xf00) >> 8;
+	asm volatile("stfpc %0" : "=m" (fp_regs->fpc));
+	vic = (fp_regs->fpc & 0xf00) >> 8;
+
 	switch (vic) {
 	case 1: /* invalid vector operation */
 		si_code = FPE_FLTINV;
--
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